Missing plaster inside at ventilation pipes.

It was also the case with my brick walls at the gable of the warm roof attic that they remained unplastered. I then considered whether I should still commission that. Various contacts, including the energy consultant, said that was unnecessary and more of an aesthetic matter. Exterior plaster apparently already provides the necessary tightness.

That is wrong and you should urgently look for new contacts. They should rather bake cakes but certainly not build or plan houses.

An exterior wall must be plastered on the inside

Why is that actually the case? I have never understood that.

According to my energy consultant, air still passes through the joints of the brick exterior walls. We even need to completely plaster the walls that connect to the exterior walls.

I will try to explain. You always have penetrations of the exterior plaster, e.g. exterior sockets or lamps. But also ventilation, extractor fans, possibly windows and window sills. Air enters the masonry somewhere. The masonry itself is not airtight and thus air can circulate throughout the entire wall.

Only a complete interior plaster prevents leaks. And there are vapor pressure tests, where a few millimeters of gap in winter can cause several liters of water to condense. This leads to mold or other problems, e.g. moisture in insulation or sockets and thus short circuits.

Ah yes, that's why the enclosed flush-mounted hollow wall boxes suitable for exterior walls are used inside. Makes sense, thank you!

Hm, and how is it with other types of houses? For example, half-timbered houses? There, only the infill panels are plastered and not the beams - that seems possible because it is not meant to be so airtight anyway? And it was (or is) also still trendy to leave masonry exposed, i.e. visible masonry. Is that only possible on interior walls?